Bitumen may by generally described as a dark-brown to black cementitious material, solid or semi-solid in consistency, in which the primary constituents are a mixture of paraffinic and aromatic hydrocarbons and heterocyclic compounds containing sulfur, nitrogen and oxygen. As discussed in the The Asphalt Handbook (The Asphalt Institute Manual, Series No. 4, 1965 ed.), incorporated herein by reference, various grades of bitumen, including coal tar and asphalt, may be produced by selecting different processing conditions. In this regard, two basic types of solid asphalt, asphalt cement and air-blown asphalt, and two basic types of liquid asphalt, cutback asphalt and emulsified asphalt, are utilized commerically. Asphalt cement is defined as asphalt which has been refined to meet paving and industrial specifications; cutback asphalt is asphalt cement which has been liquified by blending with petroleum solvents, and asphalt emulsions are prepared such that the asphalt is emulsified in the inner phase (an oil-in-water type emulsion). The emulsion can also be of the water-in-oil type in which water constitutes the inner phase (see Hellsten et al., "Asphalt Compositions Having Improved Adhesion to Aggregate," U.S. Pat. No. 3,928,061).
The particle size of mineral aggregate used in an asphalt, compositions may vary over a wide range, such as from 2.times.10.sup.-5 to 6.times.10.sup.-2 meters in diameter; or the aggregate may be of a fairly uniform size. Mineral aggregates employed in asphalt compositions also range in character from hydrophilic to hydrophobic. It has long been known that mineral aggregates have a greater attraction from water than for oil or asphalt. In general, it can be said that siliceous and acidic minerals, such as sands and gravels, tend to be very hydrophilic, whereas calcareous and alkaline materials, such as limestone, tend to be slightly hydrophilic. It is difficult, therefore, to obtain and maintain a satisfactory asphalt coating on the mineral aggregate particles when water is present. One example of an asphalt composition is the combination of asphalt cement with a size-graded mineral aggregate. This combination is referred to as asphalt concrete and is used in road paving applications. A poor asphalt coating on the mineral aggreagate leads to breakup of the asphalt concrete and commonly results in potholes and flaking pavements.
One common method of pavement construction is to remove water from the aggregate by forced evaporation prior to coating with asphalt cement. In practice this requires a certain amount of aggregate drying time which consumes energy and may result in a lengthened construction period. If weather conditions are unfavorable, such as during periods of rainfall or high humidity, road construction may be severely hindered if not halted. Even if the water is removed and the asphalt successfully desposited onto the aggregate, the asphalt coating may ultimately be degraded by the action of groundwater or rainfall.
A successful method of increasing pavement life has been to add one or more antistripping additives to the asphalt composition. Such additives increase the hydrophobicity of the aggregate, thereby strengthening and preserving the asphalt-aggregate bond.
Recently, there have been some health-related concerns with volatiles resulting from the use in asphalt of some antistripping agents currently on the market. Less volatile and less odorous antistripping agents would be desirable.